Austenitic stainless steel

ABSTRACT

A substantially nonporous, austenitic stainless steel, and a method for producing it; consisting essentially of, in weight percent, from 10 to 30% chromium, from 15 to 45% manganese, from 0.85 to 3% nitrogen, up to 1% carbon, up to 2% silicon, balance essentially iron and residuals. Moreover, a steel wherein said elements are balanced in accordance with the following equations: (30 (%C + %N) + 0.5 (%Mn))/(%Cr + 1.5 (% Si)) &gt; OR = 1.5 %Cr + 0.8 (%Mn) - 11.88 (%N - 0.1) - 28.

Hartline, III

1111 3,820,980 June 28, 1974 1 AUSTENITIC STAINLESS STEEL [75] Inventor: Albert G. Hartline, III, Tarentum,

[73] Assignee: Allegheny Ludlum Industries, Inc.,

Pittsburgh, Pa.

22 Filed: May 8,1972 [21 Appl. No.: 251,637

[52] US. Cl. 75/122, 75/126 B, 75/154 N [51] Int. Cl C22c 39/14, C22c 33/00 [58] Field of Search 75/126 B, 126 J, 134 N, 75/ 134, 122

[56] References Cited UNITED STATES PATENTS 1,378,941 5/1921 Fahrenwald 75/126 R 2,212,495 8/1940 Devries 75/126 B 2,789,048 4/1957 DeLong 75/126 B R24,431 2/1958 Jennings 75/126 B FOREIGN PATENTS OR APPLICATIONS 778,597 7/1957 Great Britain 75/126 R 892,667 3/1962 Great Britain 75/126 B Primary Examiner-Hyland Bizot Attorney, Agent, or Firm-Vincent G. Gioia; Robert F. Dropkin [5 7] ABSTRACT [30 (%C %N) +0.5 (%Mn)] /[%Cr +1.5 SU

21.5 %Cr 0.8 (%Mn) 11.88 (%N 0.1) 28.25

8 Claims, No Drawings 1 AUSTENITIC STAINLESS STEEL The present invention relates to a nonporous, high nitrogen-chromium-manganese, austenitic stainless steel, and to a method for producing it.

Today, stainless steels are available in a variety of structures exhibiting a range of mechanical properties which, combined with their excellent corrosion resistance, makes them highly versatile from a design standpoint. Of them, austenitic stainless steels generally possess the best corrosion resistance and the best strength at elevated temperatures. Austenitic stainless steels have generally been comprised of iron, chromium and nickel.

Shortages of nickel, one of the primary constituents of austenitic stainless steels, have caused considerable concern during critical times in history, and have resulted in it becoming a costly element. Out of the concern and high cost, arose extensive investigations aimed at providing austenitic steels having part or all of their nickel replaced by other elements. At the present time, the two preferred substitutions are manganese and nitrogen. The use of manganese and/or nitrogen does, however, have its drawbacks. Manganese is only half as powerful an austenitizer as is nickel and nitrogen has a tendency to produce a porous ingot.

Through the present invention, there is provided a high nitrogen-chromium-manganese, austenitic stainless steelcharacterized by high strength, good corrosion resistance and excellent ductility in the annealed condition. Moreover, an austenitic steel wherein the elements are carefully balanced to insurethe integrity of its austenitic structure and wherein sufficient chromium and manganese are present to provide a nonporous structure. The steel contains from 0.85 to 3% nitrogen, from to 30% chromium and from to 45% maganese. At first glance it appears to be somewhat similar to the steels disclosed in U.S. Pat. Nos. 2,778,731 and 2,745,740. However, the steel of US. Pat. No. 2,778,731 has a maximum equated chromium and manganese content below the minimum equated sum of chromium and manganese imposed upon the steel of the present invention and US. Pat. No. 2,745,740 does not disclose a composition balanced within the hereinbelow discussed austeniticity and porosity equation limitations imposed upon the present invention, as exemplified by the specific alloys therein. Still other references disclose relatively high nitrogen contents, but yet maximum contents below the minimum taught herein. These references are US. Pat. No. 2,909,425 and an article entitled Study of Austenilic Stainless Steels With High Manganese and Nitrogen Contents, which appeared on pages 399412 of Revue de Metallurgie, No. 5, May, 1970.

It is accordingly an object of this invention to provide a nonporous high nitrogen-chromium-manganese, austenitic stainless steel.

It is a further object of this invention to provide a method for producing a nonporous, high nitrogenchromium-manganese, austenitic stainless steel.

The nonporous, austenitic stainless steel of the present invention has a composition consisting essentially of, in weight percent, from 10 to 30% chromium, from .15 to 45% manganese, from 0.85 to 3% nitrogen, up to 1% carbon, up to 2% silicon, balance essentially iron and residuals. In addition, its elements are balanced in accordance with the following equations.

[30 (%C %N) 0.5 (%Mn)]/[%Cr 1.5 (%Si)] 1.5 (1) %ctlaaqs (%Mn) 11.88 17m A 0. 2 -25 a 0 (2) .Equation 1 is a measure of the steels austeniticity and Equation 2 is an indicator of its porosity or lack thereof. Steelswhich do not satisfy the equations are outside the scope of the invention. As a general rule, the steel of the invention is melted at an ambient pressure of about one atmosphere, and this method of making it is incorporated as a part of the present invention. The particular form in which nitrogen is added is not critical. Illustrative forms includes activated nitrogen, cyanides, and high nitrogen ferro-chrome.

Nitrogen, a strong austenitizer, is present in amounts of from 0.85 to 3 percent. At least 0.85 percent is required as it is the steel s primary strengthening element. An upper limit of 3 percent is imposed as higher nitrogen contents appear to be unrealistic from a melting standpoint. A preferred nitrogen content is from 1.05 to 1.5 percent.

Chromium is present in amounts of from 10 to 30 percent. At least 10 percent is required in order to give the steel its outstanding corrosion resistance. Chromium also has a secondary effect upon the strength of the steel and is a primary element in increasing the steels solubility for nitrogen. An upper limit of 30 percent is imposed as chromium is a ferrite former and excessive amounts of ferrite might form with higher lev els, and in turn degrade the steels properties. A preferred chromium content is from 15 to 27 percent. Steels with chromium contents below 15 percent and above 27 percent are difficult to process. Those with contents below 15 percent exhibit a greater tendency to hot short while those with contents in excess of 27 percent exhibit a greater tendency to crack during handling and forming.

Manganese is present in amounts of from 15 to 45 percent. At least 15 percent, and preferably 21 percent is necessary as manganese is an austenitizer and since manganese increases the steels solubility for nitrogen. An upper limit of 45 percent, and a preferred upper limit of 30 percent, is imposed for economic considerations, and since manganese exhibits a tendency to attack furnace refractories.

Carbon is a powerful austenitizer and strengthener and is present in amounts up to 1 percent. lts content must, however, be controlled as it can disadvantageously remove chromium from solid solution by combining therewith to form chromium carbides, and since it can reduce the steel s solubility for nitrogen by occupying interstitial sites normally filled by nitrogen. A preferred maximum carbon content is 0.15 percent. Higher carbon contents necessitate higher annealing temperatures to put the carbon into solution.

Silicon levels are maintained below 2 percent and preferably below 1 percent. Higher levels increase the inclusion content of the steel to an undesirable degree, and moreover, tie up excessive amounts of manganese inthe form of manganese silicates.

As stated above, the steel may also contain a number of residuals. These residuals include elements such as copper, molybdenum, phosphorus, sulfur, tungsten, cobalt and nickel.

500X and transmission electronmicroscopy observations at magnifications up to 50,000X. The results of this examination are also reproduced in Table II.

The following examples are illustrative of the inven- TABLE II tion.

Thirty steel heats having chromium contents from HEAT STRUCTURE 10.0 to 40.49 percent, manganese contents from 9.94 A AUSTEMTIC to 30.1 percent, n1trogen contents from 0.92 to 1.95 AUSTENITIC percent, carbon contents from 0.015 to 0.118 percent C. ALSTENITIC and silicon contents from 0.19 to 0.55 percent were 3- melted at an ambient pressure of about 1 atmosphere. AUSTENmC Their chemistry appears hereinbelow in Table I. G- S IC TABLE I CHEMISTRY HEAT C Mn P 5 st 0 Ni M0 Cu N A. 0.069 21.40 0.007 0.010 0.19 24.16 0.27 0.025 0.10 1.06 13. 0.062 25.60 0.012 0.011 0.23 25.26 0.26 0.026 0.12 1.30 c. 0.118 23.60 0.007 0.010 0.41 23.25 0.27 0.020 0.10 1.05 o. 0.068 21.50 0.006 0.011 0.51 23.22 0.25 0.025 0.24 1.11 E. 0.084 23.62 0.008 0.013 0.44 22.98 0.25 0.020 0.23 1.20 F. 0.100 21.62 0.009 0.012 0.46 24.90 0.25 0.020 0.23 1.26 o. 0.086 26.00 0.013 0.013 0.55 25.76 0.25 0.026 0.23 1.58 H. 0.033 21.40 0.009 0.010 0.52 23.26 0.32 0.010 0.24 1.45 1. 0.10 21.00 L L 0.50 25.00 0.20 0.010 0.20 1.55 J. 0.031 21.80 0.006 0.008 0.49 24.54 0.27 0.024 0.25 1.16 K. 0.10 25.00 L L 0.50 25.00 0.20 0.010 0.20 1.95 L. 0.020 25.25 0.012 0.009 0.51 24.98 0.32 0.025 0.20 1.20 M. 0.023 25.75 0.016 0.006 0.40 29.64 0.25 NA 0.19 1.03 N. 0.032 10.60 0.008 0.01 1 0.50 30.10 0.22 NA 0.21 1.04 o. 0.029 16.00 0.008 0.011 0.42 25.08 0.22 NA 0.19 1.04 P. 0.05 10.00 L L 0.50 25.00 0.20 NA 0.20 1.05 O. 0.032 25.56 0.013 0.010 0.42 29.82 0.28 NA 0.24 1.20 R. 0.054 24.50 0.010 0.009 0.39 19.84 0.26 NA 0.18 1.00 5. 0.049 20.30 0.010 0.009 0.37 20.06 0.26 NA 0.19 1.00 T. 0.05 25.00 L L 0.40 15.00 0.20 NA 0.20 1.05 u. 0.05 30.00 L L 0.40 10.00 0.20 NA 0.20 1.05 v. 0.022 10.32 0.012 0.009 0.41 35.22 0.21 NA 0.12 1.05 w. 0.028 16.65 0.011 0.010 0.38 30.29 0.20 NA 0.12 1.05 x. 0.025 29.99 0.007 0.010 0.34 15.02 0.22 NA 0.15 1.10 Y. 0019 29.84 0.008 0.006 0.51 40.34 0.29 NA 0.18 0.97 2. 0.016 30.10 0.015 0.001 0.28 35.51 0.28 NA 0.20 0.96 AA. 0.015 19.62 0.014 0.001 0.45 35.55 0.29 NA 0.18 0.93 BB. 0.015 19.61 0.016 0.001 0.44 39.79 0.29 NA 0.20 0.98 cc. 0.018 9.94 0.015 0.004 0.52 40.49 0.31 NA 0.18 1.02 DD. 0017 9.98 0.013 0.003 0.52 35.08 0.27 NA 0.20 0.92

L Low concentration requested 7 NA Analysis not performed The structure of each heat was examined. Those hav- 85882 ing chromium contents of 35 percent and more were AUSTENITIC tapped at 2,650F, sectioned and optically examined at ig g g magnifications up to 1000 X. All of them had duplex AUSTENmC structures (austenite and ferrite), as shown in Table II N. POROUS hereinbelow. Those remaining heats which were po- 2' Eggs: rous could be detected by the naked eye. They were Q, AUSTENITIC sectioned and classified porous if they had voids in exggggfi cess of one-eighth inch. Table II also shows which heats PQROUS were porous. The remaining heats were ground to reu. POROUS move casting defects, hot processed, cold processed. AUSTENlTEg FERRITE w. AUSTENITE & FERRITE and examined. I-Iot processlng 1nvolved a preheat of x, POROUS 9 o 9 Y. AUSTENITE & FERRITE 1,500 1,700 F for 1 2 hours, a heating at 2,200 2' AUSTENITE & FERRITE F for 2 3 hOUI'S, and a rolling or fOIgII'lg at 3. AA AUSTENITE & FERRITE m1n1mum temperature of 1,700 1,800F. Cold pro- 2- :Egggg O O cesslng 1nvolved an anneal at 1,900 2,000 F for 120 DD. AUSTENITE & FERRITE mmutes per inch of thlckness, an a1r cool, at least one cold roll adding up to a reduction of up to 80 percent, an anneal at 1,950F and an air cool. The examination From Table II, it is noted that heats A through G, J, involved optical observations at magnifications up to L, M, Q and R had austenitic structures, that heats H,

1, K, N through P, S through U, and X, had porous structures, and that heats V, W, and Y through DD had duplex structures of austenite and ferrite.

The carbon, nitrogen, manganese, chromium and silicon values for both the austenitic and duplex heats were inserted into the following equation, discussed hereinabove and referred to as Equation 1 therein:

(%C %N) 0.5 (%Mn)]/[%Cr 1.5 (%Si)] The calculated ratios for each of the heats is set forth below in Table III.

TABLE III CALCULATED HEAT STRUCTURE VALUE AUSTENKTIC AUSTENITIC AUSTENITIC AUSTENITIC AUSTENITIC AUSTENITIC AUSTENITIC AUSTENlTlC AUSTENlTlC AUSTENITIC AUSTENlTlC AUSTENlTIC AUSTENITE & FERRlTE AUSTENlTE & FERRITE AUSTENITE & FERRITE AUSTENITE & FERRITE AUSTENITE & FERRITE AUSTENITE & FERRlTE AUSTENITE 8t FERRlTE AUSTENITE 81. FERRITE From Table 111 it is clear that all the austenitic heats have calculated ratios in excess of 1.5, a limitation imposed upon the steel of the present invention, and that all the duplex heats (austenite and ferrite) have calculated ratiosbelow 1.5. The lowest ratio for any of the austenitic heats is 1.52 whereas the highest ratio for ny of t du lex lisa is 1.3

The chromium, manganese and nitrogen contents for both the austenitic and porous heats were inserted into the following equation, discussed hereinabove and reerrcd to a liquatiqn .2 there n;

Mn) 11.88 NO.l) -28.25

I The calculated value for each of the heats is set forth lo nT blcl L- .7

From Table IV it is clear that all the austenitic heats have calculated values in excess of 0, a limitation imposed upon the steel of the present invention, and that all the porous heats have calculated values below 0. The lowest value for any of the austenitic heats is 0.15 whereas the highest (least negative) value for any of epa eush i r011 As stated above, the properties of the steel of this invention are dependent upon the attainment of an austenitic structure. To demonstrate this, the properties of austenitic heat J are compared to those of duplex heat V, in Table V hereinbelow. No comparison is made between the properties of a porous heat and those of an austenitic heat as porous heats are obviously inferior,

and since it is near impossible to get meaningful propert ..tnsssursm msfqrthem..-

Table V compares the 0.2% yield strength, the ultimate tensile strength, the elongation and the hardness for austenitic heat .1 with duplex heat V. These proper- -ties are compared after hot rolling, after annealing at 1,950F for 7 minutes, and after cold reductions of 10, 25.8.05! 5.11%

TABLE V 7 PROPERTIES 0.2% YS Elongation HEAT STRUCTURE CONDITJQEL (psi) (psi) (70) HARDNESS .1. AUSTENITIC HOT ROLLED 177,300 190,200 23.8 46.0 RC V. AUSTENITE HOT ROLLED 75,690 108,470 17.0 97.0 Rb

8t FERRlTE 1 J. AUSTENITIC ANNEALED 118,700 157,900 44.7 33.5 RC V. AUSTENITE ANNEALED 84.680 109,810 19.0 97.0 Rb

& FERRlTE n .1. AUSTENITIC 10% COLD 140,300 177,800 29.3 41.7 RC

REDUCTION V. AUSTENiTE 10% COLD 119,560 126,030 3.5 26.0 RC

REDUCTlON & FERRITE J. AUSTENITIC 25% COLD 184400 218.200 13.8 43.7 RC

REDUCTION 1 V. AUSTENITE 25% COLD 138,50 144,390 -5 R3 REDUCTION & FERRlTE .1. AUSTENITIC COLD 231,700 269,300 7.0 48.7 RC

I REDUCTION V; v AUSTENITE 50% COLD 156,610 164,020 3.5 32.5 Rt.

& FERRITE REDUCTION From Table V, it is clear that austenitic heat J is superior to duplex heat V. Heat J had better properties than heat V after hot rolling, after annealing, and after cold rolling. Ferrite diminishes the steels yield strength, ultimate tensile strength, elongation and hardness. In addition, it detrimentally affects the steels corrosion resistance and promotes the formation of undesirable sigma phase.

The steel of this invention has utility in a wide range of applications. Included therein are high strength fasteners, motor/generator retaining rings, marine cable, and castings for pump housings.

It will be apparent to those skilled in the art that the novel principles of the invention disclosed herein in connection with specific examples thereof, will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific examples of the invention disclosed herein.

I claim:

1. A substantially nonporous, austenitic stainless steel consisting essentially of, in weight percent, from 10 to 30% chromium, from 21 to 45% manganese, from 0.85 to 3% nitrogen, up to 1% carbon, up to 2% silicon, balance essentially iron and residuals; said elements being balanced in accordance with the following equations:

6. A substantially nonporous austenitic stainless steel according to claim 1 having up to 1% silicon.

7. A substantially nonporous austenitic stainless steel according to claim 1 having from 15 to 27% chromium, from 21 to 30% manganese, from 1.05 to 1.5% nitrogen, up to 0.15% carbon and up to 1% silicon.

8. A substantially nonporous, austenitic stainless steel according to claim 1 having at least 15% chromium. 

2. A substantially nonporous, austenitic stainless steel according to claim 1 having from 15 to 27% chromium.
 3. A substantially nonporous, austenitic stainless sTeel according to claim 1 having from 21 to 30% manganese.
 4. A substantially nonporous, austenitic stainless steel according to claim 1 having from 1.05 to 1.5% nitrogen.
 5. A substantially nonporous, austenitic stainless steel according to claim 1 having up to 0.15% carbon.
 6. A substantially nonporous austenitic stainless steel according to claim 1 having up to 1% silicon.
 7. A substantially nonporous austenitic stainless steel according to claim 1 having from 15 to 27% chromium, from 21 to 30% manganese, from 1.05 to 1.5% nitrogen, up to 0.15% carbon and up to 1% silicon.
 8. A substantially nonporous, austenitic stainless steel according to claim 1 having at least 15% chromium. 